Inheritance

Type Relation

Intent

To reuse another class’s interface, implementation, and behavior with additions to or alterations of each.

Also Known As

IsA, Type Reuse

Motivation

Often, an existing class provides an excellent start for producing a new class type. The interface may be almost exactly what you are looking for, the existing methods may provide almost what you need for your new class, or, at the very least, the existing class may be conceptually close to what you wish to accomplish.

In such cases, it is useful and efficient to reuse the existing class instead of rewriting everything from scratch. One way of doing so is by copying and pasting the code into a new class. This technique is done quite often, but it has many drawbacks. If a bug is found in the original code, you now have two places to track and maintain the fix. If an enhancement is made to one copy of the code, anyone working on the other code location must be explicitly told that it exists. Copy and paste seems like a quick and easy approach in the beginning, but it doesn’t just tie the copies of code together, it ties the development teams together. In addition, in many cases, you won’t have the source code to copy at all, such as when using a vendor’s development API.